Semiconductor device

ABSTRACT

A semiconductor device in which release of oxygen from side surfaces of an oxide semiconductor film including c-axis aligned crystal parts can be prevented is provided. The semiconductor device includes a first oxide semiconductor film, a second oxide semiconductor film including c-axis aligned crystal parts, and an oxide film including c-axis aligned crystal parts. In the semiconductor device, the first oxide semiconductor film, the second oxide semiconductor film, and the oxide film are each formed using a IGZO film, where the second oxide semiconductor film has a higher indium content than the first oxide semiconductor film, the first oxide semiconductor film has a higher indium content than the oxide film, the oxide film has a higher gallium content than the first oxide semiconductor film, and the first oxide semiconductor film has a higher gallium content than the second oxide semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including anoxide semiconductor.

In this specification, a semiconductor device refers to all types ofdevices which can function by utilizing semiconductor characteristics;an electro-optical device, a semiconductor circuit, and an electronicdevice are all semiconductor devices.

2. Description of the Related Art

A technique by which a transistor is formed with a semiconductor thinfilm formed over a substrate having an insulating surface has beenattracting attention. The transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). A silicon-based semiconductor materialis widely known as a material for a semiconductor thin film applicableto the transistor. As another material, an oxide semiconductor has beenattracting attention.

For example, a transistor that includes an amorphous oxide semiconductorfilm containing indium (In), gallium (Ga), and zinc (Zn) is disclosed(see Patent Document 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

SUMMARY OF THE INVENTION

A transistor that includes an oxide semiconductor film operates faster(it can also be said that the field-effect mobility is higher) than atransistor that includes an amorphous silicon film and is manufacturedmore easily than a transistor that includes a polycrystalline siliconfilm.

However, some problems of the transistor that includes an oxidesemiconductor film have been pointed out. One of the problems isunstable electrical characteristics of the transistor. Specifically, aproblem that the threshold voltage of the transistor is negativelyshifted by a bias-temperature stress test (also referred to as a BTstress test) or irradiation with visible light or ultraviolet light, sothat the transistor tends to be normally on, has been pointed out. Asone of factors of the problem, oxygen vacancies and the like in theoxide semiconductor film can be given.

When the oxide semiconductor film is amorphous, for example, the bondingstate of metal atoms and oxygen atoms in the oxide semiconductor film isnot ordered; thus, an oxygen vacancy is generated easily. For thisreason, the electrical characteristics (e.g., electrical conductivity)of the oxide semiconductor film might be changed. This change causesvariations in the electrical characteristics of a transistor thatincludes such an oxide semiconductor film, which leads to reduction inreliability of a semiconductor device including the transistor.

Note that the oxide semiconductor film can be a single crystal state, apolycrystalline (also referred to as polycrystal) state, or the like inaddition to an amorphous state as described above. Further, as the stateof the oxide semiconductor film in which oxygen vacancies that causevariation in the electrical characteristics of the transistor can bereduced, the oxide semiconductor film can be a CAAC oxide semiconductor(also referred to as c-axis aligned crystal oxide semiconductor:CAAC-OS) film.

Here, the CAAC oxide semiconductor film will be described in detail.

The CAAC oxide semiconductor film is not absolutely amorphous. The CAACoxide semiconductor film, for example, includes an oxide semiconductorwith a crystal-amorphous mixed phase structure where crystal parts andamorphous parts are intermingled. Note that in most cases, the crystalpart fits inside a cube whose one side is less than 100 nm. From anobservation image obtained with a transmission electron microscope(TEM), a boundary between an amorphous part and a crystal part and aboundary between crystal parts in the CAAC oxide semiconductor film arenot clearly detected. Further, with the TEM, a grain boundary in theCAAC oxide semiconductor film is not clearly found. Thus, in the CAACoxide semiconductor film, reduction in electron mobility, due to thegrain boundary, is suppressed.

In each of the crystal parts included in the CAAC oxide semiconductorfilm, for example, a c-axis is aligned in a direction parallel to anormal vector of the surface where the CAAC oxide semiconductor film isformed or to a normal vector of the top surface of the CAAC oxidesemiconductor film. Further, in each of the crystal parts, metal atomsare arranged in a triangular or hexagonal configuration when seen fromthe direction perpendicular to the a-b plane, and metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that, among crystal parts, the directions of thea-axis and the b-axis of one crystal part may be different from those ofanother crystal part.

In this specification, a simple term “perpendicular” includes a rangefrom 80° to 100°, preferably from 85° to 95°. In addition, a simple term“parallel” includes a range from −10° to 10°, preferably from −5° to 5°.

In the CAAC oxide semiconductor film, distribution of crystal parts isnot necessarily uniform. For example, in the formation process of theCAAC oxide semiconductor film, in the case where crystal growth occursfrom a surface side of the oxide semiconductor film, the proportion ofcrystal parts in the vicinity of the top surface of the oxidesemiconductor film is in some cases higher than that in the vicinity ofthe surface where the oxide semiconductor film is formed. Further, whenan impurity is added to the CAAC oxide semiconductor film, the crystalpart in a region to which the impurity is added becomes amorphous insome cases.

Since the c-axes of the crystal parts included in the CAAC oxidesemiconductor film are aligned in the direction parallel to a normalvector of the surface where the CAAC oxide semiconductor film is formedor to a normal vector of the top surface of the CAAC oxide semiconductorfilm, the directions of the c-axes may be different from each otherdepending on the shape of the CAAC oxide semiconductor film (thecross-sectional shape of the surface where the CAAC oxide semiconductorfilm is formed or the cross-sectional shape of the top surface of theCAAC oxide semiconductor film). Note that the film deposition isaccompanied with the formation of the crystal parts or followed by theformation of the crystal parts through crystallization treatment such asheat treatment. Hence, the c-axes of the crystal parts are aligned inthe direction parallel to a normal vector of the surface where theCAAC-oxide semiconductor film is formed or a normal vector of thesurface of the CAAC-oxide semiconductor film.

With the use of the above-described CAAC oxide semiconductor film in atransistor, change in electrical characteristics of the transistor dueto irradiation with visible light or ultraviolet light is small. Thus,the transistor has high reliability.

In this specification, the CAAC oxide semiconductor film including thecrystal parts each having the following features is referred to as anoxide semiconductor film including c-axis aligned crystal parts: c-axesare aligned in a direction parallel to a normal vector of the surfacewhere the CAAC oxide semiconductor film is formed or to a normal vectorof the top surface of the CAAC oxide semiconductor film; triangular orhexagonal atomic arrangement which is seen from the directionperpendicular to the a-b plane is formed; and metal atoms are arrangedin a layered manner or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axis.

Moreover, in an In—Ga—Zn-based oxide (hereinafter referred to as IGZO)film which is an example of the oxide semiconductor film includingc-axis aligned crystal parts, it is evident from a computation on thebasis of the density functional theory that oxygen moves easily in aplane having an a-axis and a b-axis, whereas oxygen is difficult to movein a c-axis and an oxygen vacancy is difficult to be generated.Specifically, in arrangement of an In—O layer, a Ga—O layer, and Zn—Olayer in the IGZO film in a layered manner when seen from the directionperpendicular to the c-axis, oxygen moves along the In—O layer moreeasily than across the In—O layer. In other words, in the oxidesemiconductor film including c-axis aligned crystal parts, oxygen moveseasily along a direction parallel to the surface where the film isformed or to the top surface of the film.

In consideration of mobility of oxygen, oxygen is released from sidesurfaces of the oxide semiconductor film including c-axis alignedcrystal parts, in which case an oxygen vacancy is generated easily. Inthe case where such an oxide semiconductor film including c-axis alignedcrystal parts is processed into an island shape in the transistor thatincludes the oxide semiconductor film, the side surfaces are exposed andan oxygen vacancy is generated easily. When an oxygen vacancy iscontinued to be generated easily, variations in the electricalcharacteristics of the transistor is caused, which leads to reduction inreliability of a semiconductor device including the transistor.

Thus, one object of one embodiment of the present invention is toprovide a semiconductor device in which release of oxygen from sidesurfaces of an oxide semiconductor film including c-axis aligned crystalparts can be prevented and sufficient oxygen can be contained in theoxide semiconductor film including c-axis aligned crystal parts. Anotherobject of one embodiment of the present invention is to improve thereliability of a semiconductor device formed using a transistor thatincludes an oxide semiconductor film including c-axis aligned crystalparts.

According to one embodiment of the present invention, a semiconductordevice includes an island-like semiconductor film including a firstoxide semiconductor film and a second oxide semiconductor film includinga c-axis aligned crystal part, which is stacked over the first oxidesemiconductor film; and an oxide film including a c-axis aligned crystalpart, which is in contact with side surfaces of the island-likesemiconductor film. In the semiconductor device, the first oxidesemiconductor film, the second oxide semiconductor film, and the oxidefilm each include an oxide containing indium, gallium, and zinc, and thesecond oxide semiconductor film has a higher indium content than thefirst oxide semiconductor film, the first oxide semiconductor film has ahigher indium content than the oxide film, the oxide film has a highergallium content than the first oxide semiconductor film, and the firstoxide semiconductor film has a higher gallium content than the secondoxide semiconductor film.

According to another embodiment of the present invention, asemiconductor device includes an island-like semiconductor filmincluding a first oxide semiconductor film and a second oxidesemiconductor film including a c-axis aligned crystal part, which isstacked over the first oxide semiconductor film; an oxide film includinga c-axis aligned crystal part, which is in contact with side surfaces ofthe island-like semiconductor film; and a gate electrode provided overthe oxide film. In the semiconductor device, the first oxidesemiconductor film, the second oxide semiconductor film, and the oxidefilm each include an oxide containing indium, gallium, and zinc, and thesecond oxide semiconductor film has a higher indium content than thefirst oxide semiconductor film, the first oxide semiconductor film has ahigher indium content than the oxide film, the oxide film has a highergallium content than the first oxide semiconductor film, and the firstoxide semiconductor film has a higher gallium content than the secondoxide semiconductor film.

According to another embodiment of the present invention, asemiconductor device includes an island-like semiconductor filmincluding a first oxide semiconductor film and a second oxidesemiconductor film including a c-axis aligned crystal part, which isstacked over the first oxide semiconductor film; a source electrode anda drain electrode which are in contact with side surfaces of theisland-like semiconductor film in a channel length direction; an oxidefilm including a c-axis aligned crystal part, which is in contact withthe side surfaces of the island-like semiconductor film in a channelwidth direction; and a gate electrode provided over the oxide film. Inthe semiconductor device, the first oxide semiconductor film, the secondoxide semiconductor film, and the oxide film each include an oxidecontaining indium, gallium, and zinc, and the second oxide semiconductorfilm has a higher indium content than the first oxide semiconductorfilm, the first oxide semiconductor film has a higher indium contentthan the oxide film, the oxide film has a higher gallium content thanthe first oxide semiconductor film, and the first oxide semiconductorfilm has a higher gallium content than the second oxide semiconductorfilm.

According to the embodiment of the present invention, a side surface ofthe gate electrode is preferred to be provided with a sidewall.

According to the embodiment of the present invention, the oxide film ispreferred to have a structure in which an inorganic insulating film isstacked over a film including an oxide containing indium (In), gallium(Ga), and zinc (Zn).

According to the embodiment of the present invention, an aluminum oxidefilm is preferred to be provided over the gate electrode, the sourceelectrode, and the drain electrode.

According to the embodiment of the present invention, the first oxidesemiconductor film is preferred to be a film including an oxidecontaining In, Ga, and Zn at an atomic ratio of 1:1:1.

According to the embodiment of the present invention, the second oxidesemiconductor film is preferred to be a film including an oxidecontaining In, Ga, and Zn at an atomic ratio of 3:1:2.

According to the embodiment of the present invention, the oxide film ispreferred to be a film including an oxide containing In, Ga, and Zn atan atomic ratio of 1:3:2.

According to the embodiment of the present invention, in the crystalpart of the second oxide semiconductor film and the crystal part of theoxide film, metal atoms and oxygen atoms contained in the second oxidesemiconductor film and the oxide film are arranged in a layered manneralong a c-axis direction parallel to a normal vector of the surfacewhere the second oxide semiconductor film is formed and to a normalvector of the surface where the oxide film is formed.

According to one embodiment of the present invention, release of oxygenfrom side surfaces of an oxide semiconductor film including c-axisaligned crystal parts can be prevented and sufficient oxygen can becontained in the oxide semiconductor film including c-axis alignedcrystal parts. Moreover, according to one embodiment of the presentinvention, the reliability of a semiconductor device formed using atransistor that includes an oxide semiconductor film including c-axisaligned crystal parts can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view and FIGS. 1B and 1C are cross-sectional views,which illustrate one embodiment of a semiconductor device.

FIGS. 2A and 2B are cross-sectional views each illustrating asemiconductor device of Embodiment 1.

FIG. 3A is a plan view and FIGS. 3B and 3C are cross-sectional views,which illustrate one embodiment of a semiconductor device.

FIGS. 4A-1 to 4A-3 are plan views and FIGS. 4B-1 to 4B-3 and FIGS. 4C-1to 4C-3 are cross-sectional views, which illustrate an example of amanufacturing process of a semiconductor device.

FIGS. 5A-1 to 5A-3 are plan views and FIGS. 5B-1 to 5B-3 and FIGS. 5C-1to 5C-3 are cross-sectional views, which illustrate an example of amanufacturing process of a semiconductor device.

FIGS. 6A-1 and 6A-2 are plan views and FIGS. 6B-1 and 6B-2 and FIGS.6C-1 and 6C-2 are cross-sectional views, which illustrate an example ofa manufacturing process of a semiconductor device.

FIGS. 7A and 7B are cross-sectional views illustrating one embodiment ofa semiconductor device.

FIGS. 8A and 8B illustrate an example of circuit configuration includinga semiconductor device.

FIGS. 9A and 9B are block diagrams of a CPU including a semiconductordevice.

FIG. 10A is a plan view and FIGS. 10B and 10C are cross-sectional views,which illustrate one embodiment of a semiconductor device.

FIGS. 11A to 11C are diagrams for describing a structure of Example.

FIG. 12 is a graph for describing a structure of Example.

FIGS. 13A to 13C are diagrams for describing a structure of Example.

FIG. 14 is a graph for describing a structure of Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention disclosed in this specification will bedescribed below with reference to the accompanying drawings. Note thatthe present invention is not limited to the following description and itwill be readily appreciated by those skilled in the art that modes anddetails can be modified in various ways without departing from thespirit and the scope of the present invention. Therefore, the presentinvention should not be construed as being limited to the description inthe following embodiments.

Note that the position, size, shape, or the like of each componentillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, size, shape, or the like asdisclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a structure of a transistor that includes an oxidesemiconductor film including c-axis aligned crystal parts in asemiconductor device will be described.

FIGS. 1A to 1C illustrate a transistor of one embodiment of the presentinvention. FIG. 1A is a plan view of the transistor. FIG. 1B is across-sectional view taken along line X1-Y1 in a channel lengthdirection in FIG. 1A, and FIG. 1C is a cross-sectional view taken alongline V1-W1 in a channel width direction in FIG. 1A.

The transistor illustrated in FIGS. 1A to 1C includes an oxidation film102 over a substrate 100, a first oxide semiconductor film 104 over theoxidation film 102, a second oxide semiconductor film 106 over the firstoxide semiconductor film 104, an oxide film 108 covering at least sidesurfaces of the first island-like oxide semiconductor film 104 and thesecond island-like oxide semiconductor film 106, a gate electrode 110over the oxide film 108, an interlayer insulating film 112 covering thegate electrode 110, and a source electrode 114A and a drain electrode114B over the interlayer insulating film 112 and connected to the secondoxide semiconductor film 106. Note that the second oxide semiconductorfilm 106 includes a channel region 106A in a region overlapping with thegate electrode 110 and low-resistance regions 106B which have lowerresistance than the channel region in regions connected to the sourceelectrode 114A and the drain electrode 114B.

Note that a side surface of an island-like film is exposed perpendicularto a substrate face in one case and is tapered by being exposed in aninclined manner with respect to the substrate face in another case.

The transistor illustrated in FIGS. 1A to 1C has a structure in whichthe first island-like oxide semiconductor film 104 and the secondisland-like oxide semiconductor film 106 are stacked. According to oneembodiment of the present invention, the first oxide semiconductor film104 and the second oxide semiconductor film 106 each include an oxidecontaining at least indium, zinc, and gallium, and the second oxidesemiconductor film 106 has a higher indium content than the first oxidesemiconductor film 104. The higher indium content of the second oxidesemiconductor film 106 can lead to higher crystallinity of the secondoxide semiconductor film 106.

Moreover, according to one embodiment of the present invention, thefirst oxide semiconductor film 104 has a gallium content which is thesame as the indium content thereof and has a higher gallium content thanthe second oxide semiconductor film 106. Further, the first oxidesemiconductor film 104 can suppress diffusion of oxygen which isreleased from the oxidation film 102 at the formation of the secondoxide semiconductor film 106, silicon, or the like. As a result, byproviding the first oxide semiconductor film 104, entry of an impuritysuch as silicon into the second oxide semiconductor film 106 can bereduced, and the crystallinity of the second oxide semiconductor film106 can be improved.

For example, in the case where the first oxide semiconductor film 104 isnot formed, the second oxide semiconductor film 106 is to be directlyformed on the oxidation film 102 by thermal film formation atapproximately 400° C. In that case, oxygen is released from theoxidation film 102 before the second oxide semiconductor film 106 isformed. As a result, oxygen cannot be supplied from the oxidation film102 to the second oxide semiconductor film 106 in a later process.

In contrast, in a structure described in this embodiment in which afterthe oxidation film 102 is formed, the first oxide semiconductor film 104can be formed at a low temperature (e.g., higher than or equal to roomtemperature and lower than or equal to 200° C.) and the second oxidesemiconductor film 106 can be formed at a high temperature (e.g., higherthan or equal to 250° C. and lower than or equal to 500° C., preferablyhigher than or equal to 300° C. and lower than or equal to 400° C.), thefirst oxide semiconductor film 104 can suppress oxygen release from theoxidation film 102.

Further, the second oxide semiconductor film 106 is formed over thefirst oxide semiconductor film 104 which is formed using the same kindsof materials as the second oxide semiconductor film 106. Accordingly,the second oxide semiconductor film 106 can be a film including c-axisaligned crystal parts that grow from the interface with the first oxidesemiconductor film 104.

In other words, the first oxide semiconductor film 104 suppresses oxygenrelease from the oxidation film 102 in a manufacturing process, and alsoserves as a base film for the second oxide semiconductor film 106. As aresult, the crystallinity of the second oxide semiconductor film 106 canbe improved. After the second oxide semiconductor film 106 is formed,oxygen is released from the oxidation film 102 by heat treatment or thelike, and then the oxygen can pass through the first oxide semiconductorfilm 104 to be supplied to the second oxide semiconductor film 106.

A structure in which the first oxide semiconductor film 104 and thesecond oxide semiconductor film 106 are thus stacked has an excellenteffect of suppressing the generation of an oxygen vacancy in the secondoxide semiconductor film 106 and of improving the crystallinity of thesecond oxide semiconductor film 106.

High crystallinity of the second oxide semiconductor film 106 can makethe bonding state of metal atoms and oxygen atoms in the second oxidesemiconductor film 106 ordered, thereby suppressing the generation of anoxygen vacancy. Even though an oxygen vacancy is generated, the oxygenvacancy can be compensated with oxygen supplied from the oxidation film102.

Further, in addition to the above structure in which the first oxidesemiconductor film 104 and the second oxide semiconductor film 106 arestacked, in the transistor of one embodiment of the present invention,which is illustrated in FIGS. 1A to 1C, the oxide film 108 is providedso as to cover the side surfaces of the first island-like oxidesemiconductor film 104 and the second island-like oxide semiconductorfilm 106 including c-axis aligned crystal parts. According to oneembodiment of the present invention, as well as the second oxidesemiconductor film 106, the oxide film 108 can include c-axis alignedcrystal parts and can have an oxygen-transmitting property which islower in a perpendicular direction than in a horizontal direction to thesurface where the film is formed.

According to one embodiment of the present invention, the film having alow oxygen-transmitting property has the same elements as the firstoxide semiconductor film 104 and the second oxide semiconductor film106. In other words, in the case where the first oxide semiconductorfilm 104 and the second oxide semiconductor film 106 are IGZO films, theoxide film 108 is also an IGZO film containing indium, gallium, andzinc. In particular, the oxide film 108 has a higher gallium contentthan the first oxide semiconductor film 104 and the second oxidesemiconductor film 106 and a lower indium content than the first oxidesemiconductor film 104 and the second oxide semiconductor film 106.

Since the oxide film 108 has the same elements as the first oxidesemiconductor film 104 and the second oxide semiconductor film 106, thestate of an interface with the first island-like oxide semiconductorfilm 104 and the second island-like oxide semiconductor film 106 can befavorable. Thus, as well as the second oxide semiconductor film 106, theoxide film 108 can include c-axis aligned crystal parts.

Moreover, the oxide film 108 can have a large energy gap by having ahigher gallium content and a lower indium content than the first oxidesemiconductor film 104 and the second oxide semiconductor film 106.

Further, since the oxide film 108 as well as the first oxidesemiconductor film 104 and the second oxide semiconductor film 106contains indium, the oxide film 108 can be a film including c-axisaligned crystal parts, which is the same as the second oxidesemiconductor film 106.

Furthermore, in the oxide film 108 as well as the second oxidesemiconductor film 106 containing indium, oxygen moves easily in a planehaving an a-axis and a b-axis, whereas oxygen is difficult to move in ac-axis and an oxygen vacancy is difficult to be generated, becausec-axis aligned crystal parts are included. In arrangement of an In—Olayer, a Ga—O layer, and Zn—O layer in the above film in a layeredmanner when seen from the direction perpendicular to the c-axis, oxygenmoves along the In—O layer more easily than across the In—O layer. Theoxide film 108 can have a low oxygen-transmitting property in the c-axisdirection parallel to a normal vector of the surface where the secondoxide semiconductor film 106 is formed and to a normal vector of thesurface where the oxide film 108 is formed by utilizing a property ofoxygen which difficult to move across the In—O layer.

By providing the oxide film 108 having a low oxygen-transmittingproperty in the c-axis direction on the side surfaces of the secondoxide semiconductor film 106, a state in which oxygen is easily releasedfrom the second oxide semiconductor film 106 and an oxygen vacancy iseasily generated can be suppressed.

When the conductivity in a portion indicated by a thick dotted line inFIG. 1A is increased due to oxygen vacancies, a parasitic channel isgenerated. This parasitic channel causes reduction in switchingcharacteristics and signal delay. However, reduction in resistance ofthe portion indicated by a thick dotted line in FIG. 1A can besuppressed by providing the portion with the oxide film 108 having a lowoxygen-transmitting property in the c-axis direction. In other words, inthe cross-sectional view of FIG. 1C in the channel width direction,release of oxygen and generation of a parasitic channel in a region 116corresponding to a side surface of the second oxide semiconductor film106 can be suppressed.

In the oxide film 108, gallium functions as a stabilizer. Therefore,part or the whole of gallium can be replaced with another stabilizersuch as tin (Sn), hafnium (Hf), aluminum (Al), and zirconium (Zr) whichcan be exemplified. Further, as another stabilizer, one or plural kindsof lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), or lutetium (Lu) may be contained.

FIGS. 2A and 2B are cross-sectional views illustrating only theoxidation film 102, the first oxide semiconductor film 104, the secondoxide semiconductor film 106, and the oxide film 108.

As described above, the oxide film 108 can have a lowoxygen-transmitting property in the c-axis direction parallel to anormal vector of the surface where the film is formed by utilizing aproperty of oxygen which is difficult to move across an In—O layer. TheIn—O layer is formed along a direction parallel to the surface where thefilm is formed or to the top surface of the film; therefore, the In—Olayer can be illustrated as a layer represented by a dotted line 118 inFIG. 2A. The In—O layer represented by the dotted line 118 is providedso as to cover the side surfaces of the second island-like oxidesemiconductor film 106. Since oxygen is difficult to move across theIn—O layer, the oxide film 108 containing the In—O layer in a directionparallel to the surface where the oxide film 108 is formed or to the topsurface of the oxide film 108 can suppress release of oxygen from theside surfaces of the second oxide semiconductor film 106.

FIG. 2B is a cross-sectional view illustrating only the oxidation film102, the first oxide semiconductor film 104, the second oxidesemiconductor film 106, and the oxide film 108, whose structure isdifferent from that in FIG. 2A. FIG. 2B differs from FIG. 2A in that atrench 120 reaching the oxidation film 102 is formed in a layer of theoxidation film 102, the first oxide semiconductor film 104, and thesecond oxide semiconductor film 106, and the oxide film 108 is providedso as to cover the side surfaces of the trench 120.

A dotted line 118 of the oxide film 108 in FIG. 2B represents an In—Olayer formed along a direction parallel to the surface where the oxidefilm 108 is formed or to the top surface of the oxide film 108, in amanner similar to that of FIG. 2A. Since oxygen is difficult to moveacross the In—O layer, the oxide film 108 containing the In—O layer in adirection parallel to the surface where the oxide film 108 is formed orto the top surface of the oxide film 108 can suppress release of oxygenfrom the side surfaces of the oxidation film 102, the first oxidesemiconductor film 104, and the second oxide semiconductor film 106.

Note that when the trench remains even after the oxide film 108 isprovided so as to cover the side surfaces of the trench 120, the trenchcan be filled by forming another insulating film 122. For example, thetrench may be filled by providing an insulating film of silicon oxide orthe like. Note that polishing treatment (e.g., chemical mechanicalpolishing (CMP) treatment) may be performed for the purpose of improvingthe planarity of the surface of the insulating film 122 and exposing thesurface of the oxide film 108.

In the above-described embodiment of the present invention, the relativerelations of indium, gallium, and zinc contained in each of the firstoxide semiconductor film 104, the second oxide semiconductor film 106,and the oxide film 108 are as follows: the second oxide semiconductorfilm 106 has a higher indium content than the first oxide semiconductorfilm 104, the first oxide semiconductor film 104 has a higher indiumcontent than the oxide film 108, the oxide film 108 has a higher galliumcontent than the first oxide semiconductor film 104, and the first oxidesemiconductor film 104 has a higher gallium content than the secondoxide semiconductor film 106.

With the first oxide semiconductor film 104, the second oxidesemiconductor film 106, and the oxide film 108 having theabove-described relations, the crystallinity of the second oxidesemiconductor film 106 can be improved, and further, release of oxygenfrom the side surfaces of the second oxide semiconductor film 106including c-axis aligned crystal parts can be prevented and sufficientoxygen can be contained in the second oxide semiconductor film 106.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 2

In this embodiment, a structure of another transistor that includes anoxide semiconductor film including c-axis aligned crystal parts in asemiconductor device will be described together with a manufacturingmethod thereof, with reference to a cross-sectional view different fromthat used in the above embodiment.

FIGS. 3A to 3C illustrate another transistor of one embodiment of thepresent invention. FIG. 3A is a plan view of the transistor. FIG. 3B isa cross-sectional view taken along line X2-Y2 in a channel lengthdirection in FIG. 3A, and FIG. 3C is a cross-sectional view taken alongline V2-W2 in a channel width direction in FIG. 3A.

The transistor illustrated in FIGS. 3A to 3C includes an oxidation film202 over a substrate 200; a first oxide semiconductor film 204 over theoxidation film 202; a second oxide semiconductor film 206 includingc-axis aligned crystal parts over the first oxide semiconductor film204; a first source electrode 214A and a first drain electrode 214B incontact with side surfaces of the first island-like oxide semiconductorfilm 204 and the second island-like oxide semiconductor film 206 in thechannel length direction; an oxide film 208 which is over part of thefirst island-like oxide semiconductor film 204 and the secondisland-like oxide semiconductor film 206 and in contact with the sidesurfaces in the channel width direction; a gate electrode 210 over theoxide film 208; a sidewall 209 covering a side surface of the gateelectrode 210; an insulating film 211 covering a top surface of the gateelectrode 210; a second source electrode 213A and a second drainelectrode 213B covering top surfaces of the first source electrode 214Aand the first drain electrode 214B, a top surface of the second oxidesemiconductor film 206, and a side surface and a top of the sidewall209; and an insulating film 212 over the insulating film 211, the secondsource electrode 213A and the second drain electrode 213B, and the firstsource electrode 214A and the first drain electrode 214B. Note that thesecond oxide semiconductor film 206 includes a channel region 206A in aregion overlapping with the gate electrode 210 and low-resistanceregions 206B which have lower resistance than the channel region inregions connected to the first source electrode 214A and the first drainelectrode 214B and the second source electrode 213A and the second drainelectrode 213B.

The transistor illustrated in FIGS. 3A to 3C has a structure in whichthe formed first oxide semiconductor film 204, the second oxidesemiconductor film 206, and the oxide film 208 are stacked in a mannersimilar to that of Embodiment 1. Thus, the relative relations of indium,gallium, and zinc contained in each of the first oxide semiconductorfilm 204, the second oxide semiconductor film 206, and the oxide film208 can be made similar to those in Embodiment 1. Therefore, release ofoxygen from the side surfaces of the second oxide semiconductor film 206in the channel width direction can be prevented and sufficient oxygencan be contained in the second oxide semiconductor film 206.Accordingly, reduction in resistance of a portion indicated by a thickdotted line in FIG. 3A can be suppressed and thus generation of aparasitic channel can be suppressed.

In the structure of FIGS. 3A to 3C in this embodiment, an oxideinsulating film is used as the insulating film 212 so that theinsulating film 212 can serve as a film for preventing diffusion ofoxygen. By providing the oxide insulating film as the insulating film212, oxygen vacancies in the second oxide semiconductor film 206 can bereduced. Further, as the insulating film 212, an insulating filmincluding a metal oxide can be used. The insulating film including ametal oxide which is provided as the insulating film 212 can serve as afilm for preventing entry of hydrogen, water, or the like, which cansuppress external entry of hydrogen, water, or the like into the secondoxide semiconductor film 206 of the transistor. Accordingly, leakagecurrent of the transistor can be reduced.

Next, an example of a manufacturing process of the transistorillustrated in FIGS. 3A to 3C will be described with reference to FIGS.4A-1 to 4A-3, 4B-1 to 4B-3, and 4C-1 to 4C-3, FIGS. 5A-1 to 5A-3, 5B-1to 5B-3, and 5C-1 to 5C-3, and FIGS. 6A-1 and 6A-2, 6B-1 and 6B-2, and6C-1 and 6C-2. Note that FIGS. 4A-1 to 4A-3, FIGS. 5A-1 to 5A-3, andFIGS. 6A-1 and 6A-2 each correspond to the plan view of the transistorillustrated in FIG. 3A. FIGS. 4B-1 to 4B-3, FIGS. 5B-1 to 5B-3, andFIGS. 6B-1 and 6B-2 each correspond to the cross-sectional view takenalong line X2-Y2 illustrated in FIG. 3B. FIGS. 4C-1 to 4C-3, FIGS. 5C-1to 5C-3, and FIGS. 6C-1 and 6C-2 each correspond to the cross-sectionalview taken along line V2-W2 illustrated in FIG. 3C.

First, the oxidation film 202 is formed over the substrate 200. Theoxidation film 202 may be formed by a sputtering method, a CVD method,or the like, and is preferably formed by a method in which hydrogen,water, a hydroxyl group, hydride, and the like do not easily enter.

There is no particular limitation on a substrate that can be used as thesubstrate 200 as long as it has at least heat resistance to withstand aheat treatment step performed later. As the substrate 200, a glasssubstrate (preferably a non-alkali glass substrate), a quartz substrate,a ceramic substrate, a plastic substrate, a silicon substrate, or thelike can be used.

As the oxidation film 202, a film having an effect of preventingdiffusion of hydrogen, moisture, or the like from the substrate 200 ispreferred, which can be formed with a single-layer structure or alayered structure using one or more of a silicon oxide film, a siliconnitride oxide film, and a silicon oxynitride film.

In addition, the oxidation film 202 is preferred to be a film having aneffect of supplying oxygen to the first oxide semiconductor film 204 andthe second oxide semiconductor film 206 including c-axis aligned crystalparts, which are to be formed later, as another effect of the oxidationfilm 202. In the case where a silicon oxide film is used as theoxidation film 202, for example, part of oxygen therein can be releasedby heating the oxidation film 202, so that oxygen can be supplied to thefirst oxide semiconductor film 204 and the second oxide semiconductorfilm 206 to compensate oxygen vacancies therein.

In particular, the oxide film 202 is preferred to contain oxygen at anamount that exceeds at least the stoichiometry. For example, a siliconoxide film represented by SiO_(2+α) (α>0) is preferred to be used as theoxide film 202. With the use of such a silicon oxide film as theoxidation film 202, oxygen can be supplied to the first oxidesemiconductor film 204 and the second oxide semiconductor film 206.

Note that the planarity of the surface of the oxidation film 202 ispreferred to be improved by performing polishing treatment, dry etchingtreatment, plasma treatment, or the like. By thus improving theplanarity of the surface of the oxidation film 202, the crystallinity ofthe first oxide semiconductor film 204 and the second oxidesemiconductor film 206 which are provided over the oxidation film 202can be improved.

Next, a first oxide semiconductor film and a second oxide semiconductorfilm are formed over the oxidation film 202 and then are processed toform the first island-like oxide semiconductor film 204 and the secondisland-like oxide semiconductor film 206 (FIGS. 4A-1, 4B-1, and 4C-1).The first oxide semiconductor film 204 and the second oxidesemiconductor film 206 are formed by a method in which hydrogen, water,a hydroxyl group, hydride, and the like do not easily enter, and ispreferably formed by a sputtering method, for example.

The first oxide semiconductor film 204 and the second oxidesemiconductor film 206 are each a film including an oxide containing atleast indium, gallium, and zinc, and can be formed using an IGZO film.Note that in the IGZO film, part or the whole of gallium which is astabilizer can be replaced with another stabilizer.

The first oxide semiconductor film 204 and the second oxidesemiconductor film 206 can be formed by a sputtering method, an atomiclayer deposition (ALD) method, an evaporation method, a coating method,or the like.

The first oxide semiconductor film 204 is formed using an IGZO filmhaving a lower indium content and a higher gallium content than thesecond oxide semiconductor film 206. Moreover, the first oxidesemiconductor film 204 is formed using the IGZO film having a galliumcontent which is the same as the indium content thereof. For example, anoxide containing In, Ga, and Zn at an atomic ratio of 1:1:1 or an atomicratio close to the above atomic ratio, or an oxide containing In, Ga,and Zn at an atomic ratio of substantially 1:1:1 is used.

The thickness of the first oxide semiconductor film 204 is greater than5 nm and less than or equal to 200 nm, preferably greater than or equalto 10 nm and less than or equal to 30 nm. The first oxide semiconductorfilm 204 is in a single crystal state, a polycrystalline (also referredto as polycrystal) state, an amorphous state, or the like.

The second oxide semiconductor film 206 is formed using an IGZO filmhaving a higher indium content and a lower gallium content than thefirst oxide semiconductor film 204. Moreover, the second oxidesemiconductor film 206 is formed using the IGZO film having a higherindium content than the gallium content thereof. In other words, anoxide in which the relation of the contents can be expressed as In>Ga isused. For example, an oxide containing In, Ga, and Zn at an atomic ratioof 3:1:2 or an atomic ratio close to the above atomic ratio, or an oxidecontaining In, Ga, and Zn at an atomic ratio of substantially 3:1:2 isused.

The thickness of the second oxide semiconductor film 206 is greater than5 nm and less than or equal to 200 nm, preferably greater than or equalto 10 nm and less than or equal to 30 nm.

Further, the second oxide semiconductor film 206 includes c-axis alignedcrystal parts. In other words, the second oxide semiconductor film 206has the following features: c-axes are aligned in a direction parallelto a normal vector of the surface where the second oxide semiconductorfilm is formed or to a normal vector of the top surface of the secondoxide semiconductor film; triangular or hexagonal atomic arrangementwhich is seen from the direction perpendicular to the a-b plane isformed; and metal atoms are arranged in a layered manner or metal atomsand oxygen atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis.

Since the c-axes of the crystal parts included in the second oxidesemiconductor film 206 are aligned in the direction parallel to a normalvector of the surface where the second oxide semiconductor film 206 isformed or to a normal vector of the top surface of the second oxidesemiconductor film 206, the directions of the c-axes may be differentfrom each other depending on the shape of the second oxide semiconductorfilm 206 (the cross-sectional shape of the surface where the secondoxide semiconductor film 206 is formed or the cross-sectional shape ofthe top surface of the second oxide semiconductor film 206). Note thatwhen the second oxide semiconductor film 206 is formed, the direction ofc-axis of the crystal part is the direction parallel to a normal vectorof the surface where the second oxide semiconductor film 206 is formedor to a normal vector of the top surface of the second oxidesemiconductor film 206. The c-axis aligned crystal part is formed byfilm formation or by performing treatment for crystallization such asheat treatment after film formation.

There are three methods for forming the second oxide semiconductor film206 including c-axis aligned crystal parts. The first method is to forman oxide semiconductor film at a temperature higher than or equal to200° C. and lower than or equal to 450° C. to form, in the oxidesemiconductor film, crystal parts in which the c-axes are aligned in thedirection parallel to a normal vector of the surface where the oxidesemiconductor film is formed or to a normal vector of the top surface ofthe oxide semiconductor film. The second method is to form an oxidesemiconductor film with a small thickness and then heat it at atemperature higher than or equal to 200° C. and lower than or equal to700° C. to form, in the oxide semiconductor film, crystal parts in whichthe c-axes are aligned in the direction parallel to a normal vector ofthe surface where the oxide semiconductor film is formed or to a normalvector of the top surface of the oxide semiconductor film. The thirdmethod is to form a first oxide semiconductor film with a smallthickness, then heat it at a temperature higher than or equal to 200° C.and lower than or equal to 700° C., and form a second oxidesemiconductor film to form, in the second oxide semiconductor film,crystal parts in which the c-axes are aligned in the direction parallelto a normal vector of the surface where the second oxide semiconductorfilm is formed or to a normal vector of the top surface of the secondoxide semiconductor film.

The energy gap of the second oxide semiconductor film 206 is 2.8 eV to3.2 eV, and is greater than that of silicon, 1.1 eV. The minoritycarrier density of the second oxide semiconductor film 206 is 10 cm⁻³,which is much smaller than the intrinsic carrier density of silicon,10¹¹ cm⁻³.

Majority carriers (electrons) of the second oxide semiconductor film 206flow only from a source of the transistor. Further, the channel regioncan be depleted completely. Thus, an off-state current of the transistorcan be extremely small.

The transistor that includes the second oxide semiconductor film 206 hasa small S value, so that an ideal value can be obtained. Further, thetransistor has high reliability.

After the second oxide semiconductor film 206 is formed, the secondoxide semiconductor film 206 may be subjected to heat treatment. Thetemperature of the heat treatment is higher than or equal to 300° C. andlower than or equal to 700° C., or lower than the strain point of thesubstrate. The heat treatment can remove excess hydrogen (includingwater and a hydroxyl group) from the second oxide semiconductor film206. Note that the heat treatment is in some cases referred to asdehydration treatment (dehydrogenation treatment) in this specificationand the like.

The heat treatment can be performed in such a manner that, for example,an object to be processed is introduced into an electric furnace inwhich a resistance heater or the like is used and heated at 450° C. in anitrogen atmosphere for an hour. The second oxide semiconductor film 206is not exposed to the air during the heat treatment so that the entry ofwater and hydrogen can be prevented.

The heat treatment apparatus is not limited to the electric furnace andmay be an apparatus for heating an object to be processed by thermalconduction or thermal radiation from a medium such as a heated gas. Forexample, a rapid thermal annealing (RTA) apparatus such as a gas rapidthermal annealing (GRTA) apparatus or a lamp rapid thermal annealing(LRTA) apparatus can be used. An LRTA apparatus is an apparatus forheating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for performing heat treatment using a high-temperature gas. Asthe gas, an inert gas which does not react with an object to beprocessed by heat treatment, such as nitrogen or a rare gas such asargon is used.

For example, as the heat treatment, the GRTA process may be performed asfollows: the object to be processed is put in a heated inert gasatmosphere, heated for several minutes, and taken out of the inert gasatmosphere. The GRTA process enables high-temperature heat treatment fora short time. Moreover, the GRTA process can be employed even when thetemperature exceeds the upper temperature limit of the object to beprocessed. Note that the inert gas may be switched to a gas includingoxygen during the process.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) as its maincomponent and does not contain water, hydrogen, or the like ispreferred. For example, the purity of nitrogen or a rare gas such ashelium, neon, or argon introduced into a heat treatment apparatus isgreater than or equal to 6 N (99.9999%), preferably greater than orequal to 7 N (99.99999%) (i.e., the concentration of the impurities isless than or equal to 1 ppm, preferably less than or equal to 0.1 ppm).

The dehydration treatment (dehydrogenation treatment) might beaccompanied by release of oxygen which is a main constituent material ofthe second oxide semiconductor film 206 to lead to reduction in oxygen.An oxygen vacancy exists in a portion where oxygen is released in thesecond oxide semiconductor film 206, and a donor level which leads to achange in the electrical characteristics of the transistor is formedowing to oxygen vacancies. Therefore, in the case where the dehydrationtreatment (dehydrogenation treatment) is performed, oxygen is preferredto be supplied to the second oxide semiconductor film 206. The oxygenvacancies in the second oxide semiconductor film 206 can be compensatedwith oxygen supplied thereto.

The oxygen vacancy in the second oxide semiconductor film 206 iscompensated as follows, for example: after the second oxidesemiconductor film 206 is subjected to the dehydration treatment(dehydrogenation treatment), a high-purity oxygen gas, a high-puritydinitrogen monoxide gas, a high-purity nitrous oxide gas, or ultra dryair (the moisture amount is less than or equal to 20 ppm (−55° C. byconversion into a dew point), preferably less than or equal to 1 ppm,more preferably less than or equal to 10 ppb, in the measurement withthe use of a dew-point instrument of a cavity ring down laserspectroscopy (CRDS) system) is introduced into the same furnace. Water,hydrogen, or the like is preferred not to be contained in the oxygen gasor the dinitrogen monoxide (N₂O) gas. The purity of the oxygen gas ordinitrogen monoxide gas which is introduced into the heat treatmentapparatus is greater than or equal to 6N (99.9999%), preferably greaterthan or equal to 7N (99.99999%) (i.e., the impurity concentration in theoxygen gas or the dinitrogen monoxide gas is less than or equal to 1ppm, preferably less than or equal to 0.1 ppm).

As an example of a method for supplying oxygen to the second oxidesemiconductor film 206, oxygen (including at least any one of oxygenradicals, oxygen atoms, and oxygen ions) may be added to the secondoxide semiconductor film 206 in order to supply oxygen to the secondoxide semiconductor film 206. An ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, plasma treatment, orthe like can be employed as a method for adding oxygen.

As another example of a method for supplying oxygen to the second oxidesemiconductor film 206, oxygen may be supplied thereto in such a mannerthat the oxidation film 202 is heated to release part of oxygen. Inparticular, in this embodiment, oxygen released from the oxidation film202 is preferred to be transmitted through the first oxide semiconductorfilm 204 and supplied to the second oxide semiconductor film 206.

As described above, in the case where the second oxide semiconductorfilm 206 is formed and then the dehydration treatment (dehydrogenationtreatment) is performed to remove hydrogen or moisture from the secondoxide semiconductor film 206 so as to highly purify the second oxidesemiconductor film 206 not to contain an impurity as much as possible,the following is preferred to be performed on the second oxidesemiconductor film 206 as specific treatment: treatment for addingoxygen by which an oxygen vacancy which is generated due to reducedamount of oxygen through the dehydration treatment (dehydrogenationtreatment) is compensated by supplying oxygen to the second oxidesemiconductor film 206; or treatment for making an oxygen-excess stateby which oxygen is supplied to the second oxide semiconductor film 206so that the oxygen content of which is increased than that in thestoichiometric composition. In this specification and the like,supplying oxygen to the second oxide semiconductor film 206 may beexpressed as treatment for adding oxygen, and increasing the oxygencontent of the second oxide semiconductor film 206 than that in thestoichiometric composition may be expressed as treatment for making anoxygen-excess state.

Note that in the above-described method, the dehydration treatment(dehydrogenation treatment) and the treatment for adding oxygen may beperformed after or before the second oxide semiconductor film 206 isprocessed into an island shape. Alternatively, after the insulating film212 is formed, which is to be formed later, heat treatment may beperformed so that oxygen is supplied from the oxidation film 202 to thesecond oxide semiconductor film 206.

In this manner, hydrogen or moisture is removed from the second oxidesemiconductor film 206 by the dehydration treatment (dehydrogenationtreatment) and oxygen vacancies therein are compensated by the treatmentfor adding oxygen, whereby the second oxide semiconductor film 206 whichis of an i-type (intrinsic) or a substantially i-type (intrinsic) can beformed. The oxide semiconductor film formed in such a manner containsextremely few (close to zero) carriers derived from a donor, and thecarrier concentration thereof is lower than 1×10¹⁴/cm³, preferably lowerthan 1×10¹²/cm³, more preferably lower than 1×10¹¹/cm³.

By removing hydrogen or moisture from the second oxide semiconductorfilm 206 to highly purify the second oxide semiconductor film 206 so asnot to contain impurities as much as possible, and supplying oxygen tocompensate oxygen vacancies therein, the second oxide semiconductor film206 which is of an i-type (intrinsic) or a substantially i-type(intrinsic) can be formed. The off-state current of the transistor thatincludes the second oxide semiconductor film 206 which is highlypurified is as small as 10 yA/mm or less at room temperature, or 1 zA/mmor less at 85° C. to 95° C.

Next, a conductive film is formed to cover the first island-like oxidesemiconductor film 204 and the second island-like oxide semiconductorfilm 206 and then is processed to form the first source electrode 214Aand the first drain electrode 214B (FIGS. 4A-2, 4B-2, and 4C-2). Theprocessing may be performed by etching or the like.

As the conductive film used for the first source electrode 214A and thefirst drain electrode 214B, for example, a metal film containing anelement selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten, or a metal nitride film containing any of theabove elements as its component (e.g., a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) can be used.Alternatively, the conductive film may have a structure in which a filmof a high-melting-point metal such as titanium, molybdenum, or tungsten,or a nitride film of any of these metals (e.g., a titanium nitride film,a molybdenum nitride film, or a tungsten nitride film) is stacked oneither or both of the bottom surface and the top surface of a metal filmof aluminum, copper, or the like. Alternatively, the conductive filmused for the first source electrode 214A and the first drain electrode214B may be formed using a conductive metal oxide. As the conductivemetal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO),indium tin oxide (In₂O₃—SnO₂, which is abbreviated to ITO), or indiumzinc oxide (In₂O₃—ZnO) can be used. The conductive film used for thefirst source electrode 214A and the first drain electrode 214B can beformed using any of the above materials to have a single layer or alayered structure. There is no particular limitation on the method forforming the first source electrode 214A and the first drain electrode214B, and a variety of film formation methods such as an evaporationmethod, a PE-CVD method, a sputtering method, or a spin coating methodcan be employed.

Next, an oxide film 208 is formed to cover the second island-like oxidesemiconductor film 206 and the first source electrode 214A and the firstdrain electrode 214B (FIGS. 4A-3, 4B-3, and 4C-3).

The oxide film 208 has the same crystal structure as the second oxidesemiconductor film 206 containing indium, and can be an IGZO film here.

The oxide film 208 can be formed by a sputtering method, an atomic layerdeposition (ALD) method, an evaporation method, a coating method, or thelike.

The oxide film 208 is formed using an IGZO film having a lower indiumcontent and a higher gallium content than the first oxide semiconductorfilm 204 and the second oxide semiconductor film 206. Moreover, theoxide film 208 is formed using the IGZO film having a higher galliumcontent than the indium content thereof. In other words, an oxide inwhich the relation of the contents can be expressed as Ga>In is used.For example, an oxide containing In, Ga, and Zn at an atomic ratio of1:3:2 or an atomic ratio close to the above atomic ratio, or an oxidecontaining In, Ga, and Zn at an atomic ratio of substantially 1:3:2 isused. Further, the energy gap of the oxide film 208 can be increased byhaving a higher gallium content than the first oxide semiconductor film204 and the second oxide semiconductor film 206, whereby the oxide film208 can be used as a layer having an insulating property.

The thickness of the oxide film 208 is greater than 1 nm and less thanor equal to 500 nm, preferably greater than or equal to 10 nm and lessthan or equal to 30 nm Note that since the oxide film 208 has higherdielectric constant than an insulating film containing silicon, the filmthickness can be made thicker than the film containing silicon oranother insulating film can be stacked on the oxide film 208.

As well as the second oxide semiconductor film 206, the oxide film 208can include c-axis aligned crystal parts. In other words, the oxide film208 can have a low oxygen-transmitting property in the c-axis directionparallel to a normal vector of the surface where the film is formed byutilizing a property of oxygen which is difficult to move across an In—Olayer.

There is another method for forming the oxide film 208 as a filmincluding c-axis aligned crystal parts in a manner similar to that ofthe second oxide semiconductor 206. The first method is to form theoxide film 208 at a temperature higher than or equal to 200° C. andlower than or equal to 450° C. to form, in the oxide film 208, crystalparts in which the c-axes are aligned in the direction parallel to anormal vector of the surface where the oxide film 208 is formed or to anormal vector of the top surface of the oxide film 208. The secondmethod is to form an oxide film 208 with a small thickness, then heat itat a temperature higher than or equal to 200° C. and lower than or equalto 700° C. to form, in the oxide film 208, crystal parts in which thec-axes are aligned in the direction parallel to a normal vector of thesurface where the oxide film 208 is formed or to a normal vector of thetop surface of the oxide film 208. The third method is to form a firstoxide film 208 with a small thickness, then heat it at a temperaturehigher than or equal to 200° C. and lower than or equal to 700° C., andform a second oxide film 208 to form, in the second oxide film 208,crystal parts in which the c-axes are aligned in the direction parallelto a normal vector of the surface where the second oxide film 208 isformed or to a normal vector of the top surface of the second oxide film208.

Note that in the case where the first oxide semiconductor film 204, thesecond oxide semiconductor film 206, and the oxide film 208 which eachinclude c-axis aligned crystal parts as described above are deposited bya sputtering method, a sputtering target of a polycrystalline oxidesemiconductor is preferred to be used. When an ion collides with thesputtering target, a crystal region included in the sputtering target issometimes cleaved along an a-b plane and separated as a flat-plate-likesputtered particle or a pellet-like sputtered particle having a planeparallel to the a-b plane. In that case, the flat-plate-like sputteredparticle reaches a substrate while maintaining its crystal state,whereby a film including c-axis aligned crystal parts can be deposited.

For the deposition of the film including c-axis aligned crystal parts,the following conditions are preferred to be employed.

By reducing the amount of impurities entering the film including c-axisaligned crystal parts during the deposition, the crystal state can beprevented from being disordered by the impurities. For example, theconcentration of impurities (e.g., hydrogen, water, carbon dioxide, ornitrogen) which exist in the deposition chamber may be reduced. Further,the concentration of impurities in a deposition gas may be reduced.Specifically, a deposition gas whose dew point is lower than or equal to−80° C., preferably lower than or equal to −100° C. is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferred that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is greater than or equal to 30 vol %, preferably 100 vol%.

As an example of the sputtering target, an In—Ga—Zn—O compound targetwill be described below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y, and Z are each a givenpositive number. The kinds of powder and the molar ratio for mixingpowder may be determined as appropriate depending on the desiredsputtering target.

Next, a conductive film and an insulating film are formed over the oxidefilm 208 and then are processed to form the gate electrode 210 and theinsulating film 211. Next, a dopant is introduced into the second oxidesemiconductor film 206 using the gate electrode 210 and the insulatingfilm 211 as masks, whereby the channel region 206A and thelow-resistance regions 206B are formed in the second oxide semiconductorfilm 206 (FIGS. 5A-1, 5B-1, and 5C-1). Note that the dopant may beintroduced not only into the second oxide semiconductor film 206 butalso into the first oxide semiconductor film 204, whereby a channelregion and low-resistance regions are formed in the first oxidesemiconductor film 204.

The dopant is an impurity by which the electrical conductivity of thesecond oxide semiconductor film 206 is changed. One or more selectedfrom the following can be used as the dopant: Group 15 elements (typicalexamples thereof are nitrogen (N), phosphorus (P), arsenic (As), andantimony (Sb)), boron (B), aluminum (Al), argon (Ar), helium (He), neon(Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc(Zn).

As in this embodiment, the dopant can be introduced into the secondoxide semiconductor film 206 through the oxide film 208 by animplantation method. An ion implantation method, an ion doping method, aplasma immersion ion implantation method, or the like can be employed asa method for introducing the dopant. In that case, in addition to asingle ion of a dopant, an ion of a fluoride or a chloride of the dopantis preferred to be used. The dopant can be introduced into the secondoxide semiconductor film 206 by an implantation method without passingthrough another film.

The introduction of the dopant may be controlled by setting asappropriate the introduction conditions such as the accelerated voltageand the dosage, or the thickness of the films through which the dopantpasses. In this embodiment, phosphorus is used as the dopant, andphosphorus ions are implanted by an ion implantation method. The dosageof the dopant can be set to be greater than or equal to 1×10¹³ ions/cm²and less than or equal to 5×10¹⁶ ions/cm².

The concentration of the dopant in the low-resistance regions 206B ispreferred to be higher than or equal to 5×10¹⁸/cm³ and lower than orequal to 1×10²²/cm³.

Further, the substrate 200 may be heated while the dopant is introduced.

The introduction of the dopant into the second oxide semiconductor film206 may be performed plural times, and plural kinds of dopant may beused.

Heat treatment may be performed thereon after the dopant introduction.The heat treatment is preferably performed at a temperature higher thanor equal to 300° C. and lower than or equal to 700° C., more preferablyhigher than or equal to 300° C. and lower than or equal to 450° C.,under an oxygen atmosphere for an hour. The heat treatment may beperformed under a nitrogen atmosphere, reduced pressure, or the air(ultra-dry air).

Since the second oxide semiconductor film 206 includes c-axis alignedcrystal parts, part of the second oxide semiconductor film 206 is insome cases made amorphous by introduction of the dopant. In that case,the crystallinity of the second oxide semiconductor film 206 can berecovered by performing heat treatment thereon after the introduction ofthe dopant.

As a conductive film for forming the gate electrode 210, for example, ametal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy material includingany of these materials can be used. Alternatively, the gate electrode210 may be formed using a conductive metal oxide. As the conductivemetal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO),indium tin oxide (In₂O₃—SnO₂, which is abbreviated to ITO), or indiumzinc oxide (In₂O₃—ZnO), or any of these metal oxide materials in whichsilicon or silicon oxide is included can be used. The gate electrode 210can be formed to have a single layer or a layered structure using any ofthe above materials. There is no particular limitation on the method forforming the gate electrode 210, and a variety of film formation methodssuch as an evaporation method, a PE-CVD method, a sputtering method, ora spin coating method can be employed.

Further, as an insulating film for forming the insulating film 211, aninorganic insulating film is preferred to be used and is formed as asingle layer or a stacked layer using any of a silicon oxide film, asilicon oxynitride film, a silicon nitride film, and a silicon nitrideoxide film. There is no particular limitation on a method for formingthe insulating film 211; for example, a sputtering method, an MBEmethod, a PE-CVD method, a pulse laser deposition method, an ALD method,or the like can be employed as appropriate.

Next, an insulating film is formed to cover the gate electrode 210 andthe insulating film 211 and then is subjected to highly anisotropicetching, whereby the sidewall 209 is formed in a self-aligned manner(FIGS. 5A-2, 5B-2, and 5C-2). The insulating film for forming thesidewall 209 can be formed by a sputtering method, a CVD method, or thelike.

As an etching method for forming the sidewall 209, a dry etching methodis preferred to be employed. As an example of an etching gas used forthe dry etching method, a gas containing fluorine such astrifluoromethane, octafluorocyclobutane, or tetrafluoromethane can beused. A rare gas or hydrogen may be added to the etching gas. As the dryetching method, a reactive ion etching (RIE) method in whichhigh-frequency voltage is applied to a substrate is preferred to beemployed.

As the insulating film for forming the sidewall 209, an inorganicinsulating film is preferred to be used and is formed as a single layeror a stacked layer using any of a silicon oxide film, a siliconoxynitride film, a silicon nitride film, and a silicon nitride oxidefilm.

Next, the oxide film 208 is etched using the insulating film 211 and thesidewall 209 as masks (FIGS. 5A-3, 5B-3, and 5C-3). The oxide film 208except a region overlapping with the insulating film 211 and thesidewall 209 is removed by the etching.

Next, a conductive film is formed to cover the gate electrode 210, theinsulating film 211, the sidewall 209, the exposed second oxidesemiconductor film 206, and the first source electrode 214A and thefirst drain electrode 214B and then is processed to form the secondsource electrode 213A and the second drain electrode 213B (FIGS. 6A-1,6B-1, and 6C-1). The processing may be performed by etching or the like.

The conductive film for forming the second source electrode 213A and thesecond drain electrode 213B is favorably formed using the same materialas the first source electrode 214A and the first drain electrode 214B bya variety of film formation methods. Further, it is preferred that thethickness of the conductive film for forming the second source electrode213A and the second drain electrode 213B be made smaller than thethickness of the first source electrode 214A and the first drainelectrode 214B and that the conductive film for forming the secondsource electrode 213A and the second drain electrode 213B have highcoverage by controlling deposition rate or the like.

Next, the insulating film 212 is formed to cover the insulating film211, the second source electrode 213A and the second drain electrode213B, and the first source electrode 214A and the first drain electrode214B (FIGS. 6A-2, 6B-2, and 6C-2).

As a material for forming the insulating film 212, an inorganicinsulating film is preferred to be used and is formed as a single layeror a stacked layer using any of oxide insulating films such as a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, analuminum oxynitride film, a gallium oxide film, and a hafnium oxidefilm. Further, over the above oxide insulating film, a single layer or astacked layer of any of nitride insulating films such as a siliconnitride film, a silicon nitride oxide film, an aluminum nitride film,and an aluminum nitride oxide film may be formed. There is no particularlimitation on a method for forming the insulating film 212; for example,a sputtering method, an MBE method, a PE-CVD method, a pulse laserdeposition method, an ALD method, or the like can be employed asappropriate. In the case where an insulating film including a metaloxide is used as the insulating film 212, a metal oxide film may beformed in such a manner that a metal film is formed and then issubjected to oxygen plasma treatment or the like.

As described above, the transistor illustrated in FIGS. 3A to 3C can bemanufactured. Accordingly, a semiconductor device formed using thetransistor in which release of oxygen from the side surfaces of theoxide semiconductor films including c-axis aligned crystal parts can beprevented and sufficient oxygen can be contained therein can haveimproved reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

In this embodiment, the structure of the transistor described inEmbodiment 2 and a capacitor that can be provided in the same layer asthe transistor will be described with reference to the cross-sectionalview of FIG. 7A.

A transistor 300 illustrated in the cross-sectional view of FIG. 7Acorresponds to the transistor described in Embodiment 2 with referenceto FIGS. 3A to 3C. The transistor 300 illustrated in FIG. 7A includesthe oxidation film 202 over the substrate 200; the first oxidesemiconductor film 204 over the oxidation film 202; the second oxidesemiconductor film 206 over the first oxide semiconductor film 204; thefirst source electrode 214A and the first drain electrode 214B incontact with the side surfaces of the first island-like oxidesemiconductor film 204 and the second island-like oxide semiconductorfilm 206 in the channel length direction; the oxide film 208 which isover part of the first island-like oxide semiconductor film 204 and thesecond island-like oxide semiconductor film 206 and in contact with theside surfaces in the channel width direction; the gate electrode 210over the oxide film 208; the sidewall 209 covering the side surfaces ofthe gate electrode 210; the insulating film 211 covering the top surfaceof the gate electrode 210; the second source electrode 213A and thesecond drain electrode 213B covering the top surfaces of the firstsource electrode 214A and the first drain electrode 214B, the topsurface of the second oxide semiconductor film 206, and the side surfaceand the top of the sidewall 209; and the insulating film 212 over theinsulating film 211, the second source electrode 213A and the seconddrain electrode 213B, and the first source electrode 214A and the firstdrain electrode 214B. Note that the second oxide semiconductor film 206includes the channel region 206A in the region overlapping with the gateelectrode 210 and the low-resistance regions 206B which have lowerresistance than the channel region in the regions connected to the firstsource electrode 214A and the first drain electrode 214B and the secondsource electrode 213A and the second drain electrode 213B.

The components for forming the transistor 300 are similar to those forforming the transistor described in Embodiment 2 with reference to FIGS.3A to 3C. That is, the transistor can be formed using the oxidesemiconductor films including c-axis aligned crystal parts, in whichrelease of oxygen from the side surfaces of the oxide semiconductorfilms including c-axis aligned crystal parts can be prevented andsufficient oxygen can be contained therein.

For a capacitor 301, the components of the transistor 300 can be used.Specifically, an electrode layer 302 which forms one electrode of thecapacitor 301 can be formed using the same material as the first sourceelectrode 214A and the first drain electrode 214B.

An insulating film 303 of the capacitor 301 can be formed using the samematerial as the oxide film 208.

An electrode layer 304 which forms the other electrode of the capacitor301 can be formed using the same material as the gate electrode 210.

An insulating film 305 of the capacitor 301, which is formed over theelectrode layer 304, can be formed using the same material as theinsulating film 211.

An insulating film 306 of the capacitor 301, which is formed on a sidesurface of the electrode layer 304, can be formed using the samematerial as the sidewall 209.

The insulating film 303 of the capacitor 301 can be formed using thesame material as the oxide film 208. In other words, the insulating film303 has a lower indium content and a higher gallium content than thefirst oxide semiconductor film 204 and the second oxide semiconductorfilm 206. Moreover, the oxide film 208 is formed using an IGZO filmhaving a higher gallium content than the indium content thereof,specifically, a film including an oxide containing In, Ga, and Zn at anatomic ratio of 1:3:2 or an atomic ratio close to the above atomicratio. The insulating film 303 including the oxide can have a dielectricconstant as high as approximately 15 compared with an insulating filmcontaining silicon such as silicon oxynitride. Therefore, largeelectrostatic capacitance of the capacitor 301 can be obtained and thusthe size of the capacitor 301 can be reduced.

Next, in FIG. 7B, the structure of a memory device which includes thetransistor 300 and the capacitor 301 described in FIG. 7A and which canhold stored data even when not powered and has no limitation on thenumber of write cycles will be described.

The memory device illustrated in FIG. 7B includes a lower element layer321 including an n-channel transistor 331 and a p-channel transistor 332whose channel regions are formed using a silicon material and an upperelement layer 324 including the transistor 300 and capacitor 301described in FIG. 7A, which is electrically connected to the lowerelement layer 321 through a wiring layer 322 and a wiring layer 323.

The n-channel transistor 331 in FIG. 7B includes an SOI layer 335provided over a substrate 333 including a semiconductor material (e.g.,silicon) with a BOX layer 334 provided therebetween, n-type impurityregions 336 formed in the SOI layer 335, a gate insulating film 337, anda gate electrode 338. Although not illustrated, the SOI layer 335includes intermetallic compound regions and a channel region in additionto the n-type impurity regions 336. In the p-channel transistor 332,p-type impurity regions 339 are formed in an SOI layer 335.

An element isolation insulating layer 342 is provided between the SOIlayers 335 of the n-channel transistor 331 and the p-channel transistor332, and an insulating film 340 is provided to cover the n-channeltransistor 331 and the p-channel transistor 332. Note that in then-channel transistor 331 and the p-channel transistor 332, with the useof sidewalls formed on side surfaces of the gate electrodes, regionshaving different concentrations of impurities may be included in then-type impurity regions 336 and the p-type impurity regions 339.Further, a wiring 341 is provided in the insulating film 340 over then-type impurity regions 336 and the p-type impurity regions 339, aninsulating film 344 in the wiring layer 322 and an insulating film 345in the wiring layer 323.

The n-channel transistor 331 and the p-channel transistor 332 which eachinclude the SOI layer 335 including a semiconductor material can beoperated at high speed. Therefore, with the use of the transistors asreading transistors of the memory device, data can be read at highspeed. The transistor 300 and the capacitor 301 is favorably formed insuch a manner that the top surface of the wiring 341 is exposed bysubjecting the top surface of the wiring layer 323 to CMP treatment.

As described above, in the semiconductor device in this embodiment, thetransistors whose channel regions are formed using silicon and thetransistor whose channel region is formed using the oxide semiconductorfilm including c-axis aligned crystal parts, which is described above inEmbodiment 1, can be provided by being stacked. As a result, a space foreach element can be saved and thus the size of the semiconductor devicecan be reduced.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, an example of a mode in which another structure isadded to the transistor described in Embodiment 1 will be described.

FIGS. 10A to 10C illustrate another transistor of one embodiment of thepresent invention. FIG. 10A is a plan view of the transistor. FIG. 10Bis a cross-sectional view taken along line X3-Y3 in a channel lengthdirection in FIG. 10A, and FIG. 10C is a cross-sectional view takenalong line V3-W3 in a channel width direction in FIG. 10A.

The transistor illustrated in FIGS. 10A to 10C includes the oxidationfilm 102 over the substrate 100, the first oxide semiconductor film 104over the oxidation film 102, the second oxide semiconductor film 106over the first oxide semiconductor film 104, the oxide film 108 coveringthe first island-like oxide semiconductor film 104 and the secondisland-like oxide semiconductor film 106, an insulating film 401 overthe oxide film 108, a gate electrode 110 over the insulating film 401,the interlayer insulating film 112 covering the gate electrode 110, andthe source electrode 114A and drain electrode 114B which are over theinterlayer insulating film 112 and connected to the second oxidesemiconductor film 106. Note that the second oxide semiconductor film106 includes the channel region 106A in the region overlapping with thegate electrode 110 and the low-resistance regions 106B which have lowerresistance than the channel region in the regions connected to thesource electrode 114A and drain electrode 114B.

The transistor illustrated in FIGS. 10A to 10C is different from thetransistor illustrated in FIGS. 1A to 1C in that the insulating film 401is included. It is preferred that the insulating film 401 be aprotective film having a shielding effect, which prevents penetration ofboth oxygen and an impurity such as hydrogen or moisture into the secondoxide semiconductor film 106.

As the insulating film 401, an inorganic insulating film is preferred tobe used and can be formed as a single layer or a stacked layer using anyof oxide insulating films such as a silicon oxide film, a siliconoxynitride film, an aluminum oxide film, an aluminum oxynitride film, agallium oxide film, and a hafnium oxide film. Further, over the aboveoxide insulating film, a single layer or a stacked layer of any ofnitride insulating films such as a silicon nitride film, a siliconnitride oxide film, an aluminum nitride film, and an aluminum nitrideoxide film may be formed. For example, as a stacked layer, a siliconoxide film and an aluminum oxide film can be stacked from the gateelectrode 110 side by a sputtering method. There is no particularlimitation on a method for forming the insulating film 401; for example,a sputtering method, an MBE method, a PE-CVD method, a pulse laserdeposition method, an ALD method, or the like can be employed asappropriate.

Alternatively, a particularly dense inorganic insulating film can beformed as the insulating film 401. For example, an aluminum oxide filmcan be formed by a sputtering method. By forming an aluminum oxide filmhaving high density (a film density of 3.2 g/cm³ or higher, preferably3.6 g/cm³ or higher), a high shielding effect (blocking effect) ofpreventing penetration of both oxygen and an impurity such as hydrogenor moisture into the second oxide semiconductor film 106, can beobtained. Therefore, in and after the manufacturing process, thealuminum oxide film functions as a protective film for preventing animpurity such as hydrogen or moisture, which causes variation in theelectrical characteristics of the transistor, from being mixed into thesecond oxide semiconductor film 106 and for preventing oxygen from beingreleased, which is a main constituent material of the second oxidesemiconductor film 106. Note that the film density can be measured byRutherford backscattering spectrometry (RBS) or X-ray reflection (XRR).

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 5

In this embodiment, an example of a circuit configuration of a memorydevice which is formed using the transistor whose channel region isformed using an oxide semiconductor film including c-axis alignedcrystal parts, which is described above in Embodiment 1, and which canhold stored data even when not powered and has no limitation on thenumber of write cycles will be described with reference to drawings.

The circuit configuration in FIGS. 8A and 8B is an example in whichstored data can be held even when not powered and there is no limitationon the number of write cycles.

In FIG. 8A, a first wiring (1st Line) is connected to one of a sourceelectrode and a drain electrode of a transistor 801. A second wiring(2nd Line) is connected to the other of the source electrode and thedrain electrode of the transistor 801. A third wiring (3rd Line) isconnected to one of a source electrode and a drain electrode of thetransistor 802. A fourth wiring (4th Line) is connected to a gateelectrode of the transistor 802. Further, a gate electrode of thetransistor 801, the other of the source electrode and the drainelectrode of the transistor 802, and one electrode of a capacitor 803are connected to one another. A fifth wiring (5th Line) is connected tothe other electrode of the capacitor 803.

In the figures, “OS” is written to indicate that the transistor 802 is atransistor whose channel region is formed using an oxide semiconductorfilm including c-axis aligned crystal parts, which is described in theabove embodiments.

The circuit configuration in FIG. 8A utilizes the advantage that thepotential of the gate electrode of the transistor 801 can be held,whereby writing, holding, and reading of data can be performed asdescribed below.

Writing and holding of data are described. First, the potential of thefourth wiring is set to a potential at which the transistor 802 isturned on, so that the transistor 802 is turned on. Accordingly, thepotential of the third wiring is supplied to the gate electrode of thetransistor 801 and to the capacitor 803. In other words, a predeterminedcharge is supplied to the gate electrode of the transistor 801 (i.e.,writing of data). Here, one of two kinds of charge providing differentpotentials (hereinafter referred to as a low-level charge and ahigh-level charge) is given. After that, the potential of the fourthwiring is set to a potential at which the transistor 802 is off, so thatthe transistor 802 is turned off. Thus, the charge supplied to the gateelectrode of the transistor 801 is held (i.e., holding of data).

Since the off-state current of the transistor 802 is extremely small,the charge of the gate electrode of the transistor 801 is held for along time.

Next, reading of data is described. By supplying an appropriatepotential (reading potential) to the fifth wiring with a predeterminedpotential (constant potential) supplied to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld in the gate electrode of the transistor 801. This is because ingeneral, when the transistor 801 is an n-channel transistor, an apparentthreshold voltage V_(th) _(_) _(H) in the case where the high-levelelectric charge is given to the gate electrode of the transistor 801 islower than an apparent threshold voltage V_(th) _(_) _(L) in the casewhere the low-level electric charge is given to the gate electrode ofthe transistor 801. Here, the apparent threshold voltage refers to thepotential of the fifth wiring, which is needed to turn on the transistor801. Thus, by setting the potential of the fifth wiring to a potentialV₀ which is between V_(th) _(_) _(H) and V_(th) _(_) _(L), charge givento the gate electrode of the transistor 801 can be determined. Forexample, in the case where a high-level charge is given in writing, whenthe potential of the fifth wiring is set to V₀ (>V_(th) _(_) _(H)), thetransistor 801 is turned on. In the case where a low-level charge isgiven in writing, even when the potential of the fifth wiring is set toV₀ (<V_(th) _(_) _(L)), the transistor 801 remains in an off state.Therefore, the stored data can be read by the potential of the secondwiring.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells need to be read. In the memory cell wheredata are not read, a potential at which the transistor 801 is turned offregardless of the state of the gate electrode, that is, a potentiallower than V_(th) _(_) _(H) may be supplied to the fifth wiring.Alternatively, a potential at which the transistor 801 is turned on,that is, a potential higher than V_(th) _(_) _(L) may be supplied to thefifth wiring regardless of the state of the gate electrode.

When a transistor whose channel region is formed using an oxidesemiconductor film including c-axis aligned crystal parts, which hasextremely small off-state current, is applied to the memory devicehaving the circuit configuration shown in this embodiment, the memorydevice can hold data for an extremely long period. In other words, powerconsumption can be sufficiently reduced because refresh operationbecomes unnecessary or the frequency of refresh operation can beextremely low. Moreover, stored data can be held for a long period evenwhen power is not supplied (note that a potential is preferred to befixed).

Further, in the memory device having the circuit configuration shown inthis embodiment, high voltage is not needed for writing data and thereis no problem of deterioration of elements. For example, unlike aconventional non-volatile memory, it is not necessary to inject andextract electrons into and from a floating gate, and thus a problem suchas deterioration of a gate insulating layer does not occur at all. Inother words, the memory device in this embodiment does not have thelimitation on the number of writing, which is a problem of aconventional nonvolatile memory, and the reliability thereof issignificantly improved. Further, data are written depending on the onstate and the off state of the transistor, whereby high-speed operationcan be easily achieved.

Note that the transistor 801 includes a semiconductor layer formed usingsilicon, and the transistor 802 includes the second oxide semiconductorfilm 106 including c-axis aligned crystal parts. In other words, thetransistor 801 and the transistor 802 can be provided by being stackedas described in Embodiment 3. As a result, even when the transistor 801and the transistor 802 differ from each other in size, increase in thesize of the memory device can be suppressed.

Next, in FIG. 8B, an example of the circuit configuration in whichstored data can be held even when not powered and there is no limitationon the number of write cycles, which is different from the circuitconfiguration of FIG. 8A, will be described.

In the circuit configuration of a memory cell 810 shown in FIG. 8B, abit line BL is connected to one of a source electrode and a drainelectrode of a transistor 811. A word line WL is connected to a gateelectrode of the transistor 811. The other of the source electrode anddrain electrode of the transistor 811 is connected to one electrode of acapacitor 812.

The transistor 811 that includes an oxide semiconductor film includingc-axis crystal parts has extremely small off-state current. For thatreason, the potential of one electrode of the capacitor 812 (or chargeaccumulated in the capacitor 812) can be held for an extremely long timeby turning off the transistor 811.

Next, writing and holding data in the memory cell 810 in FIG. 8B aredescribed.

First, the potential of the word line WL is set to a potential at whichthe transistor 811 is turned on, so that the transistor 811 is turnedon. Accordingly, the potential of the bit line BL is supplied to the oneelectrode of the capacitor 812 (i.e., writing of data). After that, thepotential of the word line WL is set to a potential at which thetransistor 811 is turned off, so that the transistor 811 is turned off.Thus, the potential of the one electrode of the capacitor 812 is held(i.e., holding of data).

Since the off-state current of the transistor 811 is extremely small,the potential of the one electrode of the capacitor 812 (or the chargeaccumulated in the capacitor 812) can be held for a long time.

Next, reading of data is described. When the transistor 811 is turnedon, the bit line BL which is in a floating state and the capacitor 812are electrically connected to each other, and the charge isredistributed between the bit line BL and the capacitor 812. As aresult, the potential of the bit line BL is changed. The amount ofchange in the potential of the bit line BL varies depending on thepotential of the one electrode of the capacitor 812 (or the chargeaccumulated in the capacitor 812).

For example, the potential of the bit line BL after chargeredistribution is (C_(B)*V_(B0)+C*V)/(C_(B)+C), where V is the potentialof the one electrode of the capacitor 812, C is the capacitance of thecapacitor 812, C_(B) is the capacitance of the bit line BL (hereinafteralso referred to as bit line capacitance), and V_(B0) is the potentialof the bit line BL before the charge redistribution. Therefore, it canbe found that assuming that the memory cell 810 is in either of twostates in which the potentials of the one electrode of the capacitor 812are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case ofholding the potential V₁ (=(C_(B)*V_(B0)+C*V₁)/(C_(B)+C)) is higher thanthe potential of the bit line BL in the case of holding the potential V₀(=(C_(B)*V_(B0)+C*V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predeterminedpotential, data can be read.

As described above, the circuit configuration illustrated in FIG. 8B canhold charge accumulated in the capacitor 812 for a long time because theoff-state current of the transistor 811 is extremely small. In otherwords, power consumption can be sufficiently reduced because refreshoperation becomes unnecessary or the frequency of refresh operation canbe extremely low. Moreover, stored data can be held for a long periodeven when power is not supplied.

This embodiment can be implemented in appropriate combination with anyof the other embodiments. Therefore, the memory device formed using thetransistor that includes the oxide semiconductor film including c-axisaligned crystal parts can have higher reliability.

Embodiment 6

In this embodiment, a structure example of a nonvolatile flip-flopincluding a pair of a volatile memory portion formed using a transistorwhose channel region is formed using silicon and a nonvolatile memoryportion formed using the transistor whose channel region is formed usingan oxide semiconductor film including c-axis aligned crystal parts,which is described above in Embodiment 1, will be described. With one ormore such nonvolatile flip-flops, a nonvolatile register that can storeone-bit or multi-bit data can be obtained.

FIG. 9A shows an example of a block diagram of a nonvolatile registerthat can store n-bit data. A nonvolatile register 900 shown in FIG. 9Aincludes n nonvolatile flip-flops 901.

The nonvolatile flip-flop 901 includes a volatile memory portion 902 anda nonvolatile memory portion 903.

The volatile memory portion 902 includes a flip-flop 904. In FIG. 9A, aD-flip-flop is shown as an example of the flip-flop 904. Power issupplied from a high power supply potential VDD and a low power supplypotential GND to the flip-flop 904 of the volatile memory portion 902,and a clock signal CLK and data D_1 to D_n are input into the flip-flop904 thereof. Besides, a signal for inputting and outputting data,performing initialization, or the like may be input into the flip-flop904 depending on its circuit configuration. The data D_1 to D_n inputinto a terminal D of the flip-flop 904 are held and output from outputterminals Q_1 to Q_n in synchronization with the clock signal CLK.

Note that the flip-flop 904 is formed using a plurality of transistorswhose channel regions are formed using silicon. The flip-flop 904 isformed using a miniaturized transistor so as to read or write data athigh speed.

The nonvolatile memory portion 903 includes a transistor 905 whosechannel region is formed using an oxide semiconductor film and acapacitor 906. In the nonvolatile memory portion 903 shown in FIG. 9A,the capacitor 906 can be charged and discharged with a charge by turningon the transistor 905 by a control signal WE, and in the nonvolatilememory portion 903 shown in FIG. 9A, the charge held in the capacitor906 is held by turning off the transistor 905 by a control signal WE.Even when power is not supplied, the charge can be held in the capacitor906 in accordance with the logic state of data by utilizing an extremelysmall leakage current of the transistor 905.

Note that the transistor 905 corresponds to the transistor whose channelregion is formed using an oxide semiconductor, which is described abovein Embodiment 1. Therefore, in the transistor 905, release of oxygenfrom side surfaces of the oxide semiconductor films including c-axisaligned crystal parts can be prevented and sufficient oxygen can becontained therein, whereby the reliability of the nonvolatile register900 can be improved.

Next, a specific mode in the case where the nonvolatile register is usedfor a CPU will be described. An example of a block diagram of a CPU andperipheral circuits thereof are illustrated in FIG. 9B.

A CPU 950 includes a controller portion 951 and an arithmetic unitportion 952. In FIG. 9B, as the peripheral circuits of the CPU 950, adata buffer circuit 953, a power source control circuit 954, a powerswitching circuit 955, and an internal control signal generation circuit956 are shown.

The controller portion 951 includes a data latch circuit 957, aninstruction register circuit 958, a control circuit 959, a registergroup 960, and an address buffer circuit 961. The control circuit 959includes a state machine 962. The register group 960 includes a programcounter 963, a general purpose register circuit 964, and an arithmeticregister circuit 965. The arithmetic unit portion 952 includes anarithmetic logic unit (ALU) 966.

Data, an address, and a control signal are input and output into andfrom each circuit of the CPU and the peripheral circuits thereof via anaddress bus and a control bus in addition to a data bus. Note that inFIG. 9B, the data bus is indicated by a heavy line, the control bus isindicated by a thin line, and the address bus is omitted.

The data buffer circuit 953 is a buffer memory circuit that temporarilystores data including an instruction (program) which is input and outputinto and from the controller portion 951. The power source controlcircuit 954 controls supply of power in the power switching circuit 955depending on a control signal input from the outside and outputs acontrol signal WE for controlling a nonvolatile register included ineach circuit of the controller portion 951. The power switching circuit955 switches whether to supply or not power input from the outsidedepending on the control of the power source control circuit 954. Theinternal control signal generation circuit 956 outputs a clock signalCLK for controlling the nonvolatile register included in each circuit ofthe controller portion 951 depending on the control of the power sourcecontrol circuit 954.

The data latch circuit 957 temporarily stores data including aninstruction (program) which is input and output into and from thecontroller portion 951 and then selectively supplies the data to eachcircuit of the controller portion 951 via the data bus. The instructionregister circuit 958 temporarily stores instruction data transmitted tothe controller portion 951. The control circuit 959 has a function ofdecoding the input instruction and making each circuit of the controllerportion 951 execute the instruction. Further, the state machine 962 ofthe control circuit 959 temporarily stores the state of the controllerportion 951. The program counter 963 of the register group 960 stores anaddress of an instruction to be executed next. The general purposeregister circuit 964 of the register group 960 temporarily stores dataread from an external main memory device. The arithmetic registercircuit 965 of the register group 960 temporarily stores data which areobtained during arithmetic processing of the ALU 966. The address buffercircuit 961 temporarily stores and outputs an address of an instructionto be executed next to the external main memory device. The ALU 966 ofthe arithmetic unit portion 952 has a function of performing a varietyof arithmetic operations such as four arithmetic operations and logicoperations.

Next, the operation of the CPU 950 will be described.

In response to an address of an instruction to be executed, the CPU 950accesses a corresponding address of the main memory device via theaddress buffer circuit 961. Then, the instruction is read from theexternal main memory device and stored in the instruction registercircuit 958.

The CPU 950 decodes and executes the instruction stored in theinstruction register circuit 958. Specifically, when arithmeticprocessing is performed on the decoded instruction, the control circuit959 generates various signals for controlling the operation of the ALU966 in response to the decoded instruction. The ALU 966 performsarithmetic processing using data stored in the general purpose registercircuit 964 and temporarily stores the data obtained by the arithmeticprocessing in the general purpose register circuit 964 or the arithmeticregister circuit 965. In the case of storing or reading data, the CPU950 accesses as appropriate the external main memory device or eachcircuit of the register group 960 in response to the decodedinstruction.

Note that in the CPU 950 shown in FIG. 9B, the instruction registercircuit 958, the control circuit 959, the register group 960, and theaddress buffer circuit 961 of the controller portion 951, whichtemporarily store data, each include the above-described nonvolatileregister. In other words, the data of the instruction register circuit958, the control circuit 959, the register group 960, and the addressbuffer circuit 961 of the controller portion 951 are not erased evenwhen the supply of power is stopped, and the state where data isrestored can be obtained when power is supplied again. As a result,power consumption can be reduced in the case where rereading of data inthe CPU 950 or the supply of power is not needed.

This embodiment can be implemented in appropriate combination with anyof the other embodiments. Therefore, the CPU described in thisembodiment can be formed using the transistor that includes the oxidesemiconductor film including c-axis aligned crystal parts and thus ahighly reliable CPU can be obtained.

Example 1

In this example, in an IGZO film which is a ternary metal oxide as anexample of the oxide semiconductor film including c-axis aligned crystalparts, the results of computations of an excess oxygen atom (an oxygenatom whose proportion is in excess of the proportion of oxygen instoichiometry) mobility and oxygen vacancy mobility will be described.

Note that in the computation, a model in which one excess oxygen atom orone oxygen vacancy exists in one In—O plane of one IGZO (312) plane isformed by structure optimization (see FIGS. 11A to 11C and FIGS. 13A to13C), and each energy of intermediate structures along a minimum energypath was calculated by a nudged elastic band (NEB) method.

The computation was performed using calculation program software“OpenMX” based on the density functional theory (DFT). Parameters aredescribed below.

As a basis function, a pseudoatom local basis function was used. Thebasis function is classified into polarization basis sets STO (slatertype orbital).

As a functional,generalized-gradient-approximation/Perdew-Burke-Ernzerhof (GGA/PBE) wasused.

The cut-off energy was 200 Ry.

The sampling point k was 5×5×3.

In the computation of mobility of an excess oxygen atom, the number ofatoms which existed in the computation model was set to 85, and in thecomputation of oxygen vacancy mobility, the number of atoms whichexisted in the computation model was set to 83.

Mobility of an excess oxygen atom and mobility of an oxygen vacancy areevaluated by calculation of a height Eb of energy barrier which isrequired to go over in moving to respective sites. In other words, whenthe height Eb of energy barrier which is gone over in moving is high, anexcess oxygen atom or an oxygen vacancy hardly moves, and when theheight Eb of the energy barrier is low, excess oxygen or an oxygenvacancy easily moves.

First, movement of an excess oxygen atom is described. FIGS. 11A to 11Cshow models used for computation of movement of an excess oxygen atom.The computations of two transition forms described below were performed.FIG. 12 shows the computations results. In FIG. 12, the horizontal axisindicates a path length (of movement of an excess oxygen atom), and thevertical axis indicates energy (required for the movement) with respectto energy in a state of a model A in FIG. 11A.

In two transition forms of the movement of an excess oxygen atom, afirst transition is a transition from the model A to a model B, and asecond transition is a transition from the model A to a model C.

In FIGS. 11A to 11C, an oxygen atom denoted by “1” is referred to as afirst oxygen atom of the model A; an oxygen atom denoted by “2” isreferred to as a second oxygen atom of the model A; and an oxygen atomdenoted by “3” is referred to as a third oxygen atom of the model A.

As seen from FIG. 12, the maximum value (Eb_(max)) of the height Eb ofthe energy barrier in the first transition is 0.53 eV, and that of thesecond transition is 2.38 eV. That is, the maximum value (Eb_(max)) ofthe height Eb of the energy barrier in the first transition is lowerthan that of the second transition. Accordingly, energy required for thefirst transition is smaller than energy required for the secondtransition, and the first transition occurs more easily than the secondtransition.

In other words, the first oxygen atom of the model A moves more easilyin the direction in which the second oxygen atom of the model A ispushed than in the direction in which the third oxygen atom of the modelA is pushed. Accordingly, this shows that the oxygen atom moves moreeasily along a layer of indium atoms than across the layer of indiumatoms.

Next, oxygen vacancy movement is described. FIGS. 13A to 13C show modelsused for computation of oxygen vacancy movement. The computations of twotransition forms described below were performed. FIG. 14 shows thecomputations results. In FIG. 14, the horizontal axis indicates a pathlength (of oxygen vacancy movement), and the vertical axis indicatesenergy (required for the movement) with respect to energy in a state ofa model A in FIG. 13A.

In the two transition forms of the oxygen vacancy movement, a firsttransition is a transition from the model A to a model B, and a secondtransition is a transition from the model A to a model C.

Note that dotted circles in FIGS. 13A to 13C represent oxygen vacancies.

As is seen from FIG. 14, the maximum value (Eb_(max)) of the height Ebof the energy barrier of the first transition is 1.81 eV, and that ofthe second transition is 4.10 eV. That is, the maximum value (Eb_(max))of the height Eb of the energy bather of the first transition is lowerthan that of the second transition. Accordingly, energy required for thefirst transition is smaller than energy required for the secondtransition, and the first transition occurs more easily than the secondtransition.

That is, the oxygen vacancy of the model A moves more easily to theposition of an oxygen vacancy of the model B than to the position of anoxygen vacancy of the model C. Accordingly, this shows that the oxygenvacancy also moves more easily along a layer of indium atoms than acrossthe layer of indium atoms.

Next, in order to compare probabilities of occurrence of theabove-described four transition forms from another aspect, temperaturedependence of these transitions is described. The above-described fourtransition forms are (1) the first transition of an excess oxygen atom,(2) the second transition of the excess oxygen atom, (3) the firsttransition of an oxygen vacancy, and (4) the second transition of theoxygen vacancy.

Temperature dependence of these transitions is compared with each otherbased on movement frequency per unit time. Here, movement frequency Z(per second) at certain temperature T (K) is represented by thefollowing formula (1) when the number of vibrations Zo (per second) ofan oxygen atom in the chemically stable position is used.

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack & \; \\{Z = {{Zo} \cdot {\exp \left( {- \frac{{Eb}_{\max}}{kT}} \right)}}} & (1)\end{matrix}$

Note that in the formula (1), Eb_(max) represents the maximum value ofthe height Eb of an energy barrier of each transition, and k representsa Boltzmann constant. Further, Zo=1.0×10¹³ (per second) is used for thecalculation.

In the case where an excess oxygen atom or an oxygen vacancy moves onceper second (in the case of Z=1 (per second)) beyond the maximum value(Eb_(max)) of the height Eb of the energy barrier, when the formula (1)is solved for T, the following formulae are obtained:

(1) In the first transition of an excess oxygen atom where Z=1, T=206 K(−67° C.);(2) In the second transition of the excess oxygen atom where Z=1, T=923K (650° C.);(3) In the first transition of an oxygen vacancy where Z=1, T=701 K(428° C.); and(4) In the second transition of the oxygen vacancy where Z=1, T=1590 K(1317° C.).

On the other hand, Z in the case where T=300 K (27° C.) is as follows:

(1) In the first transition of an excess oxygen atom where T=300 K,Z=1.2×10⁴ (per second);(2) In the second transition of the excess oxygen atom where T=300 K,Z=1.0×10⁻²⁷ (per second);(3) In the first transition of an oxygen vacancy where T=300 K,Z=4.3×10⁻¹⁸ (per second); and(4) In the second transition of the oxygen vacancy where T=300 K,Z=1.4×10⁻⁵⁶ (per second).

Further, Z in the case where T=723 K (450° C.) is as follows:

(1) In the first transition of an excess oxygen atom where T=723 K,Z=2.0×10⁹ (per second);(2) In the second transition of the excess oxygen atom where T=723 K,Z=2.5×10⁻⁴ (per second);(3) In the first transition of an oxygen vacancy where T=723 K, Z=2.5(per second); and(4) In the second transition of the oxygen vacancy where T=723 K,Z=2.5×10⁻¹⁶ (per second).

In view of the above-described calculation, in the case where eitherT=300 K or T=723 K, an excess oxygen atom moves more easily along alayer of indium atoms than across the layer of indium atoms. Moreover,in the case where either T=300 K or T=723 K, an oxygen vacancy alsomoves more easily along the layer of indium atoms than across the layerof indium atoms.

Further, in the case where T=300 K, the movement of the excess oxygenatom along the layer of indium atoms occurs extremely easily; however,the other transitions do not occur easily. In the case where T=723 K,not only the movement of the excess oxygen atom along the layer ofindium atoms but the movement of the oxygen vacancy along the layer ofindium atoms occurs easily; however, either the excess oxygen atom orthe oxygen vacancy is difficult to move across the layer of indiumatoms.

Thus, for example, as in the oxide semiconductor film including c-axisaligned crystal parts, in the case where a layer of indium atoms existson a plane parallel to the surface where the oxide semiconductor film isformed or to the top surface of the oxide semiconductor film, either anexcess oxygen atom or an oxygen vacancy moves easily along the surfacewhere the oxide semiconductor film is formed or the top surface of theoxide semiconductor film.

As described above, it was evident from the computations in this examplethat, in the oxide semiconductor film including c-axis aligned crystalparts, an excessive oxygen atom and an oxygen vacancy easily moved alongthe surface where the oxide semiconductor film was formed or the topsurface of the oxide semiconductor film. In consideration of suchmobility of oxygen, it was evident that oxygen was easily released fromside surfaces of the oxide semiconductor film including c-axis alignedcrystal parts and thus an oxygen vacancy was easily generated.Accordingly, oxygen is easily released from a side surface of the oxidesemiconductor film. In the case where the oxide semiconductor filmincluding c-axis aligned crystal parts is processed into an island shapein the transistor that includes such an oxide semiconductor film, theside surfaces are exposed and oxygen vacancies are generated easily. Asdescribed in the above embodiments, according to one embodiment of thepresent invention, the oxygen vacancies can be reduced and reduction inreliability of a semiconductor device that includes the oxidesemiconductor film including c-axis aligned crystal parts can besuppressed.

Note that the case where the excess oxygen atom or the oxygen vacancymoves across the layer of indium atoms is described above; however, thepresent invention is not limited thereto, and the same applies to layersof metals other than indium which are contained in the oxidesemiconductor film.

This application is based on Japanese Patent Application serial No.2012-020510 filed with the Japan Patent Office on Feb. 2, 2012, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an island-likesemiconductor film comprising: a first oxide semiconductor film; and asecond oxide semiconductor film including a c-axis aligned crystal part;an oxide film adjacent to the second oxide semiconductor film interposedbetween the first oxide semiconductor film and the oxide film, whereinthe oxide film comprises a c-axis aligned crystal part; a gate electrodeadjacent to the oxide film interposed between the island-likesemiconductor film and the gate electrode; and a source electrode and adrain electrode electrically connected to the island-like semiconductorfilm, wherein the first oxide semiconductor film, the second oxidesemiconductor film, and the oxide film each include an oxide containingindium, gallium, and zinc, wherein an indium content in the second oxidesemiconductor film is higher than an indium content in the first oxidesemiconductor film, wherein the indium content in the first oxidesemiconductor film is higher than an indium content in the oxide film,wherein a gallium content in the oxide film is higher than a galliumcontent in the first oxide semiconductor film, and wherein the galliumcontent in the first oxide semiconductor film is higher than a galliumcontent in the second oxide semiconductor film.